1. Field of the Invention
The invention relates to a control device of an engine with a turbocharger, and a method of controlling the engine.
2. Description of Related Art
As one type of engine installed on a vehicle, such as an automobile, an engine supercharged with air by means of an exhaust gas turbocharger is in practical use. In this type of engine, there is a certain relationship between increase of output torque and increase of the boost pressure, and, even if a throttle valve is opened so as to increase the output torque, it is difficult to quickly increase the output torque since the boost pressure rises with a delay in response to opening of the throttle valve. Therefore, even if a required value of output torque required to be generated from the engine is increased, due to an increase of the amount of accelerator operation by the driver of the vehicle, the output torque of the engine may not necessarily be increased in good response to the increase of the required value.
For example, when the required value of the output torque for the engine increases as indicated by a two-dot chain line in FIG. 6A, the throttle valve is, opened so as to increase the output torque of the engine according to the increased required value. If the throttle valve is opened, air and fuel supplied to combustion chambers are increased to some extent immediately after the opening of the throttle valve. However, the air and fuel subsequently supplied to the combustion chambers are only gradually increased since the boost pressure is not immediately increased. Therefore, the output torque of the engine based on combustion of the fuel in the combustion chambers initially increases as indicated by a solid line in period t1 of FIG. 6A, and then the increase of the output torque stagnates (i.e., slows down or stops) as indicated by the solid line in period t2. However, if the engine speed increases and the flow amount of exhaust gas of the engine increases as the fuel burns in the combustion chambers, the turbocharger is driven by the exhaust gas, and the boost pressure rapidly increases as indicated in FIG. 6B. If the throttle valve is placed in an open state under a situation where the boost pressure is rapidly increased, the amounts of air and fuel supplied to the combustion chambers are quickly increased, and the output torque of the engine based on combustion of the fuel quickly increases in period t3 of FIG. 6A.
In Japanese Patent Application Publication No. 2008-14281 (JP 2008-14281 A), it is proposed to initially suppress output torque of an engine, so as to curb a sudden change or changes in the output torque of the engine from the time when the turbocharger is started to the time when the turbocharger operates with good responsiveness. With the technology of JP 2008-14281 A applied, the output torque of the engine is initially suppressed (in periods t1, t2) before the turbocharger shows sufficient responsiveness and the boost pressure rises as indicated in period t3 of FIG. 6B.
As a result, the output torque of the engine varies as indicated by a broken line in FIG. 6A by way of example, and a sudden change in the output torque is less likely or unlikely to appear in periods t1, t2. At the time of transition from period t2 to period t3, the output torque of the engine increases smoothly.
If the output torque of the engine is initially suppressed before the turbocharger shows sufficient responsiveness, as described in JP 2008-14281 A, a sudden change in the output torque of the engine is less likely or unlikely to appear during a period from the time when the turbocharger is started to the time when the turbocharger shows sufficient responsiveness. However, if the output torque is suppressed as described above when a required value of the output torque for the engine increases due to an increase of the amount of accelerator operation by the driver, the output torque of the engine is inevitably reduced in an early period following the increase of the required value. In this case, the output torque of the engine is not increased as intended by the driver even if the driver increases the accelerator operation amount. Therefore, the driver may feel slowness or sluggishness in increase of the output torque.
If the output torque is not suppressed when the required value of the output torque for the engine increases with increase of the amount of accelerator operation by the driver, the driver will not feel uncomfortable or strange due to the slowness in the increase of the output torque. In this case, however, as the boost pressure rapidly increases at the time of transition from period t2 to period t3 in FIG. 6B, the output torque of the engine rapidly increases as indicated by the solid line in period t3 of FIG. 6A, from a condition where the increase of the output torque stagnates as indicated by the solid line in period t2. Consequently, the driver may feel strange or uncomfortable due to the rapid increase in the output torque of the engine.